In recent years, in wireless communication systems, such as mobile phone systems or the like, in order to implement high-speed and high-capacity wireless communication, the next-generation wireless communication technology is being discussed. For example, at the 3rd Generation Partnership Project (3GPP) that is a standards organization, the communication standard named Long Term Evolution (LTE) and the communication standard named LTE-A (LTE-Advanced) based on the LTE wireless communication technology are standardized.
In LTE-A, as one of the technologies of increasing the capacity of systems, Multi-user Multiple Input Multiple Output (MU-MIMO) is supported, in which a plurality of user signals are transmitted through the same radio resource by using a spatial multiplexing technique.
Furthermore, in LTE Release 8, transmission to each mobile terminal device (User Equipment: UE) is limited to the number of ranks of 1 and the precoding weight is selected from a code book that is previously defined. For example, by using a downlink control channel, a wireless base station (evolutional Node B: eNB) notifies the UE of the Precoding Matrix Index (PMI) that is the information on the selected precoding weight. By using the notified PMI information and Cell-specific Reference Signals (CRS), the UE demodulates a data signal (physical downlink shared channel: PDSCH) addressed to the UE.
Furthermore, in LTE Release 9, the number of ranks per UE is increased to 2 and dedicated pilot signals (DM-RS) with a maximum of 2 layers can be inserted. Furthermore, with LTE in Release 10, DM-RSs with a maximum of 8 layers can be inserted. With LTE in Release 9 and the subsequent release, because DM-RS subjected to the same precoding as that performed on a data channel is used, there is no constraint to select a precoding weight from a code book.
Consequently, an eNB can use arbitrary precoding; does not need to notify each UE of the PMI and send, as a notification, an antenna port number of the DM-RS that is allocated to the own eNB. UE modulates the PDSCH by using the notified DM-RS. Furthermore, with LTE in Release 10, as a downlink DM-RS, 8 antenna ports are defined and spatial multiplexing transmission of a signal with a maximum of 8 layers is supported.
Furthermore, with LTE-A, in order to reduce interference between cells and in order to improve received signal intensity, the introduction of a Coordinated MultiPoint (CoMP) communication technology is being studied. With CoMP communication technology, multiple communication points that are geographically separated each other perform communication in cooperation with each other. Each of the communication points is constituted by, for example, a base station, a femto base station, a remote antenna, or the like. By using these multiple communication points, transmission is adjusted between multiple points.
For example, for downlink CoMP, Joint Transmission (JT), Dynamic Point Selection (DPS), and Coordinated Scheduling/Coordinated Beamforming (CS/CB) are being studied. The JT is a method of simultaneously transmitting the same data from multiple communication points to a wireless terminal. The DPS is a method of dynamically selecting a point, from among the multiple communication points, from which data is actually transmitted. The CS/CB is a method of performing precoding control in coordination between multiple points.
In the JT or the DPS, multiple coordinated communication points transmit a data signal to the same UE. In the CS/CB, each coordinated cell transmits data to UE in its own cell. Furthermore, for uplink CoMP, a method (Joint Reception: JR) of joining signals received at multiple points while communication is being performed between points and a method, such as DPS, is being studied.    Non Patent Literature 1: 3GPP TS36.211 V10.4.0 (2011-12)    Non Patent Literature 2: 3GPP TS36.212 V10.5.0 (2012-3)    Non Patent Literature 3: 3GPP TS36.213 V10.5.0 (2012-3)    Non Patent Literature 4: 3GPP TS36.214 V10.1.0 (2011-3)    Non Patent Literature 5: 3GPP TR36.814 V9.0.0 (2010-03)
However, in the conventional technology, because suppression of interfere is not sufficiently performed, there is a problem in that the communication quality is degraded.
For example, in LTE Release 9 and beyond, downlink MU-MIMO transmission using DM-RS is standardized. In Release 11, standardization of downlink CoMP transmission has been developed. In the downlink MU-MIMO transmission, signals addressed to a plurality of UEs are multiplexed into the same radio resource. Furthermore, in the downlink CoMP transmission in the CS/CB, data transmission to UE in its own cell is performed between coordinated cells by using the same radio resource.
In both methods, an eNB selects a combination of UEs, in which PMIes that are fed back from UEs and that are orthogonal to each other, or adjusts precoding. However, as described above, with a measure against interference at the time of transmission, it is difficult to sufficiently remove an interference signal from a received signal and thus, the communication quality of the signals that are decoded by each UE and that are addressed to its own device is low.